In the distant murky past, Andreas Koenig wrote CPAN.pm
At YAPC::Europe 2001, Jos Boumans and Michael G Schwern talked about CPANTS, which lead to a discussion on how CPAN.pm would need to be changed to support CPANTS. Jos offered to look in to it. He spoke to Andreas and asked for advice, and also chatted with many other people about what they wanted with CPAN.pm and CPAN.
Instead of patching CPAN.pm, Jos decided it would be easier to rewrite it in a modular fashion. The multiple layers make it possible for many changes to take place while the API remains consistent, enabling others to build upon it. Plug-ins are also possible with the design.
Jos was joined by Joshua Boschert and Autrijus Tang. A number of people have worked on CPANPLUS, including Joshua Keroes and Michael G Schwern. Chromatic recently joined and is putting together a new test suite.